In the production of an ethylene polymer in the presence of a catalyst containing a transition metal, the use of a catalyst obtained by supporting a chromium compound on a carrier makes it possible to produce an ethylene polymer having properties which cannot be attained by other catalysts such as Ziegler catalyst. Thus, such a catalyst is generally known as a representative ethylene polymerization catalyst comparable to Ziegler catalyst.
It is known that the use of a so-called Phillips catalyst comprising chromium trioxide supported on a solid inorganic oxide such as silica makes it possible to produce a blow-moldable ethylene polymer having an excellent moldability. However, the Phillips catalyst is disadvantageous in that it undergoes no effect of a molecular weight modifier such as hydrogen as generally used for Ziegler catalyst and thus exhibits a poor hydrogen response. Thus, ethylene polymers which can be produced in the presence of Phillips catalyst are limited to those having a molecular weight falling within a relatively high range.
As described in JP-B-45-40902 (The term "JP-B" as used herein means an "examined Japanese patent publication"), a catalyst comprising chromocene supported on a solid inorganic oxide such as silica undergoes a good effect of hydrogen as a molecular weight modifier and thus exhibits a good hydrogen response that allows the production of ethylene polymers having a relatively low molecular weight. However, this catalyst is disadvantageous in that it has a low activity and hence gives a poor productivity.
As described in JP-B-44-2996, JP-B-47-1766, JP-B-47-20004 and JP-B-47-21574, a catalyst obtained by reducing a chromic acid ester supported on a solid inorganic oxide such as silica with an organic aluminum compound, too, undergoes the effect of hydrogen as a molecular weight modifier. However, such a catalyst is disadvantageous in that it exhibits a deteriorated hydrogen response as compared with the foregoing catalyst comprising chromocene supported in a carrier and an insufficient activity.
An object of the present invention is to provide a process for the efficient production of an ethylene polymer having a wide molecular weight range at a remarkably improved productivity given by eliminating the above-described difficulties and attaining a high activity and a good hydrogen response.